Is there any information about which caching proxies the famous CDNs are using?
Are they using OS proxies for caching or commercial?
Some use lighttpd or nginx. Some use squid or varnish.
Get something like headerspy for Firefox, surf a few and check the headers that are returned.
Akamai uses a modified version of Squid.
Akamai caches (courtesy Neal Cardwell)
The akamai machines at the edge are PCs running Linux and a slightly modified version of the squid cache. They are doing on-demand caching rather than push-based replication.